The factors in cytosol controlling coenzyme dissociation, particularly of tyrosine aminotransferase are being defined. Dephosphorylation of pyridoxal-P appears not to be a major factor. There appears to be an additional factor affecting the enzyme in the cytosol fraction. Work proceeds to identify this component. Somatic cell genetics is being used to determine the nature of the expression of tyrosine aminotransferase.